linux-kernel - Re: [PATCH v6 0/6] arm64: Add kernel probes (kprobes) support

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Message-ID: <555370B9.2070203@redhat.com>
Date:	Wed, 13 May 2015 11:41:45 -0400
From:	William Cohen <wcohen@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	David Long <dave.long@...aro.org>,
	Will Deacon <will.deacon@....com>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	"sandeepa.s.prabhu@...il.com" <sandeepa.s.prabhu@...il.com>,
	Steve Capper <steve.capper@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 0/6] arm64: Add kernel probes (kprobes) support

On 05/13/2015 05:22 AM, Masami Hiramatsu wrote:
> On 2015/05/12 21:48, William Cohen wrote:

>> Hi Dave,
>>
>> In some of the previous diagnostic output it looked like things would go wrong
>> in the entry.S when the D bit was cleared and the debug interrupts were 
>> unmasksed.  I wonder if some of the issue might be due to the starting the 
>> kprobe for the trampoline, but leaving things in an odd state when another
>> set of krpobe/kretporbes are hit when the trampoline is running.
> 
> Hmm, does this mean we have a trouble if a user kprobe handler calls the
> function which is probed by other kprobe? Or, is this just a problem
> only for kretprobes?

Hi Masami,

I wrote an example based off of sample/kprobes/kprobes_sample.c to force the reentry issue for kprobes (the attached kprobe_rentry_example.c). That seemed to run fine.  I think the reason that the trampoline handler got into trouble is because of the reset_current_kprobe() before the possible call to kfree, but I haven't verified it. It seems like that should be at the end of trampoline handler just before the return.  Other architectures have similar trampoline handlers, so I am surprised that the other architectures haven't encountered this issue with kretprobes.  Maybe this is due to specific of arm64 exception handling.

# modprobe kprobe_reentry_example
[  909.617295] Planted kprobe at fffffe00000b7b34
[  909.623873] Planted kprobe at fffffe000032d34c
# rmmod kprobe_reentry_example
[ 1482.647504] kprobe at fffffe00000b7b34 unregistered
[ 1482.687506] kprobe at fffffe000032d34c unregistered
[ 1482.692361] y = 42
[ 1482.694361] z = 0
# grep \ int_sqrt$ /proc/kallsyms 
fffffe000032d34c T int_sqrt
# grep \ do_fork$ /proc/kallsyms 
fffffe00000b7b34 T do_fork

> 
>>  As Dave
>> mentioned the proposed trampoline patch avoids using a kprobe in the
>> trampoline and directly calls the trampoline handler.  Attached is the
>> current version of the patch which was able to run the systemtap testsuite.
>>  Systemtap does exercise the kprobe/kretprobe infrastructure, but it would
>> be good to have additional raw kprobe tests to check that kprobe reentry
>> works as expected.
> 
> Actually, Will's patch looks like the same thing what I did on x86,
> as the kretprobe-booster. So I'm OK for that. But if the above problem
> is not solved, we need to fix that, since kprobes can be used from
> different sources.

The patch should look similar to the x86 code. The x86 code was used as a model.

-Will


View attachment "kprobe_reentry_example.c" of type "text/x-csrc" (2828 bytes)